Non-cyclic phosphodiesters, which are generally found to be present at the millimolar level in tissues from a wide variety of organisms, constitute a relatively abundant class of phosphorus-containing metabolites. Glycerophosphoryl choline (GPC) and glycerophosphoryl ethanolamine (GPE) are most frequently observed in mammalian tissues, however serine ethanolamine phosphodiester (SEP) and threonine ethanolamine phosphodiester (TEP) analogs are found in other species such as chickens and fish, respectively, as a consequence of the relatively high abundance of these compounds and the presence of an NMR detectable phosphorus nucleus in a unique spectral range, in vivo 31p NMR provides a unique opportunity for studying the role of these compounds. An extensive series of NMR measurements has suggested that a principal function of these compounds may be to act as inhibitors of lysolecithianse, and in turn as regulators of membrane composition and structure. During the past year, enzymatic studies were carried out demonstrating that 50% inhibition of rat liver lysolecithianse is observed at 1mM and 5mM levels of GPC and SEP, respectively. Further studies of the effects of food and water deprivation on phosphodiester content and lipid composition of chicken kidney were also carried out. The SEP content of the kidney was found to more than double after 24 hours, and preliminary lipid analyses suggest a loss of phospholipid. We have recently noted that GPC levels are extremely high in semen. Studies of samples from a dog model of muscular dystrophy indicate significant changes in GPC levels, as well as marked phosphodiesterase activity. Further studies on the relationship of these changes are in progress.